Method of making microcrystalline fluoride fibers

ABSTRACT

A process for producing metal fluoride fibers by impregnating organic precursor fibers with a soluble metal salt, precipitating insoluble fluoride of the metal within the precursor fibers by introducing a second solution comprising a fluoride which when reacted with said first salt will yield said insoluble metal fluoride plus another second salt which can be subsequently removed from the precursor fiber without removing said metal fluoride, the second salt thus produced being either soluble so that it can be removed by washing, or else being capable of gassification at temperatures low enough to avoid degradation of the desired metal fluoride, and the organic precursor then being degraded and driven off by a carefully controlled sintering operation.

United States Patent [191 Abrams et al.

[451 Aug. 27, 1974 METHOD OF MAKING MICROCRYSTALLINE FLUORIDE FIBERS [75] Inventors: Edwin F. Abrams, Silver Spring,

Md.; Robert G. Shaver, Fairfax, Va.

[22] Filed: Mar. 26, 1973 [21] Appl. No.: 344,884

[52] U.S. Cl 423/489, 423/163, 423/490 [51] Int. Cl..... C01b 9/08, C0lf 11/22, COlg 53/08 [58] Field of Search 423/489, 490, 155, 163

[56] References Cited UNITED STATES PATENTS 2,526,584 10/1950 Shenk 423/489 3,385,915 5/1968 Hamling 106/55 X 3,529,929 9/l970 Page et al. 423/490 FOREIGN PATENTS OR APPLICATIONS 540,076 10/ 1941 Great Britain 423/490 1,238,069 7/1971 Great Britain 423/490 9/1967 France 423/490 1/1967- U.S.S.R 423/490 Primary ExaminerEdward Stern Attorney, Agent, or FirmAlexander & Dowell [57] ABSTRACT A process for producing metal fluoride fibers by impregnating organic precursor fibers with a soluble metal salt, precipitating insoluble fluoride of the metal within the precursor fibers by introducing a second solution comprising a fluoride which when reacted with said first salt will yield said insoluble metal fluoride plus another second salt which can be subsequently removed from the precursor fiber without removing said metal fluoride, the second salt thus produced being either soluble so that it can be removed by washing, or else being capable of gassification at temperatures low enough to avoid degradation of the desired metal fluoride, and the organic precursor then being degraded and driven off by a carefully con trolled sintering operation.

5 Clains, No Drawings METHOD OF MAKING MICROCRYSTALLINE FLUORIDE FIBERS GOVERNMENT CONTRACT The invention herein described was made under a Contract with the Department of the Air Force.

This invention relates to a novel and improved method of making small diameter microcrystalline metal fluoride reinforcement fibers by conversion of organic textile-type fibers, usually resembling textiletype fibers. These fluoride fibers are especially useful as reinforcements in certain matrix materials, such as fluoroelastomers, which are themselves useful in making seals for liquid propellant engines wherein the materials in the seals must be compatible with the liquid propellant.

BACKGROUND This invention provides an improved way of making fluoride reinforcement fibers according to a process which, in part, includes steps that are employed also in the prior art for the conversion of filaments and fibers or yarns for the purpose of making other types of inorganic fibers, for instance, incandescent mantles. For example, U.S. Pat. Nos. 768,073; 987,333; 1,035,527; 1,436,359 and 1,601,746 teach processes for converting textile fabrics into inorganic fiber mantles of corresponding shape by treating the organic fibers with certain chemical compounds so as to precipitate inorganic salts within the organic fibers, and subsequently buming off the organic fiber materials which remain after such precipitation. However, these are older processes which, while providing inorganic fibers having excellent incandescent properties, produced fibers suffering from very poor mechanical properties including poor strength and excessive brittleness. Such properties could be tolerated in a lamp mantle which is subjected to very little mechanical stress tending to cause failure, but the present invention is concerned with the production of inorganic fibers to be used as strengthening materials requiring superior mechanical properties as well as chemical compatibility with the matrix materials in which they are to be used. In some respects, U.S. Pat. No. 3,385,915 comes closer to the present invention because the metal oxide fibers being made by its process of conversion are useful for their mechanical properties, as distinguished from their incandescence when heated. However, the process of U.S. Pat. No. 3,385,915 differs from the present invention in some important respects attributable to the fact that oxide fibers are being made instead of fluoride fibers. After impregnation of the organic fibers with the desired salt solutions in that patent oxidation is desirable to convert the impregnated metal salt into an oxide, and this need goes hand-in-hand with the need to degrade and remove the organic material of the original fiber. Conversely, inthe present invention, oxidation of the metallic compounds is deleterious and must be avoided since fluoride fibers, not oxide fibers, are desired. Hence, in U.S. Pat. No. 3,385,915 the metal compound which is introduced into the fiber by soaking is not precipitated by introduction of another chemical compound also by soaking, but instead the compound firstintroduced in that patent is directly converted to the ultimate oxide by the heat which is used also to degrade the organic material of the original fiber. Thus, the patented process makes a fiber using a process which differs from the presently disclosed process and also pro duces a different final product.

INVENTION It is the principal object of this invention to provide a process of conversion of an organic fiber into an inorganic fiber with particular regard to the chemical composition and the crystalline structure of the resulting inorganic fiber for the purpose of providing the latter with compatibility and with optimum strength. The fibers produced in this manner have very small diameters, up to approximately 15 microns depending partly upon the diameter of the precursor fibers with which the process is started. The length of the resulting fibers also depends upon the lengths of the precursor fibers used. The organic precursor fibers or filaments should be of a type having the capability of absorbing aqueous -solutions in which they are to be soaked, and for this purpose textile-type fibers such as cotton or rayon have been found particularly useful. Thus, the range of precursor materials corresponds essentially with the ranges recited in the above mentioned patents.

The precursor fibers are soaked in water for a substantial period of time to cause them to swell in the manner suggested in these prior patents, and then they are soaked in a first metal compound solution to impregnate the fiber with the first compound. After maximum impregnation, the fibers are then soaked in a second different compound comprising a fluoride salt, probably although not necessarily, in the form of an aqueous solution, these two compounds having been selected so that they react within the fiber to precipitate out within the precursor the metal fluoride of which the ultimate inorganic fiber will be made. There are certain requirements which must be met in selecting these two compounds when they are to be impregnated by soaking into the organic precursor. These conditions are (1) that both of the chemical compounds with which the precursor is impregnated must be soluble in a solvent which will swell but not attack the precursor fiber, and they must contain the chemical elements which are necessary to react with each other for the purpose of precipitating out the metal fluoride of which the ultimate inorganic fiber will be made; and (2) that the metal fluoride of which the inorganic fiber will be made must be stable and non-melting at the elevated temperatures which are necessary in the final step wherein the temperature of the fibers is raised to the temperature necessary to degrade the organic fiber and leave only the inorganic metal fiber which is the end product of the present process. When the two impregnating compounds react to precipitate out the desired metal fluoride, they also form another compound as a result of the reaction and this compound is also formed within the precursor fiber and must be removed therefrom. The present process contemplates several ways of accomplishing its removal depending upon its nature. Usually this other compound can be one that is soluble and can therefore be washed from the precursor fiber, leaving the insoluble metal fluoride still therewithin; or alternatively it can be a compound that will gassify and be driven off as the heating step required to degrade the organic precursor proceeds. In any event, however, the chemical compounds required to carry out this process must (3) react to form a metal fluoride and another compound which can be separated from the precursor and the precipitated fluoride without removing or deteriorating the latter.

It is another principal object of this invention to teach ways of degrading the organic fibers, after the reacting chemical compounds have been impregnated thereinto and the metal fluoride has been precipitated, in such a way as to avoid oxidizing of the metal while at the same time producing optimum microcrystalline structure in the final fluoride fiber which provides it with superior tensile strength. The manner in which the heat is applied to the fibers and the lengths of time required for it to build up, and to be sustained, are important from the point of view of providing optimum crystalline structure to be sure that the resulting structure is microcrystalline rather than macrocrystalline. Column 7 beginning at line 14 of the above mentioned US. Pat. No. 3,385,915 discusses in detail the fact that heating of the impregnated precursor must be done in slow steps in order to permit the mechanical shrinkage of the decomposing organic fiber material to take place in a manner promoting the formation of microcrystalline structure in the remaining inorganic fiber, in order to prevent mechanical fracturing of the inorganic fiber, in order to prevent ignition of the fiber material and/or melting of the metal compound, and in order to avoid excessive crystallization and grain growth thereof. The present process must follow this procedure also not only for the reasons given, but also in such a way as to avoid oxidation of the metal present, which oxidation is affirmatively sought in the aforementioned patent. The patent recognizes that there is a certain amount of oxidizing agent present in the organic precursor itself. In the present disclosure, this agent is given a long time in which to combine with materials other than the metal at the lower temperatures by starting at about 100C. and increasing to about 400C. slowly over about a four hour period of time, and then slowly continuing to higher temperatures as described below in connection with the examples given. Moreover, oxidation can be avoided altogether by performing the degrading in a fluorine gas atmosphere whereby atmospheric oxidizing agents are excluded altogether. However, the heating must still follow the above mentioned gradual pattern in order to avoid the mechanical problems outlines above and obtain the desired microcrystalline structure.

As described in a number of prior-art textile fiber conversion patents, suitable absorbent precursor fibers, yarns or fabrics are made of a wide variety of organic materials such as natural cotton or wood fibers, or a wide variety of man-made organic fibers such as polyesters, etc., althoughrayon is especially suitable due to its structural uniformity. Others are described in the fourth column of US. Pat. No. 3,385,915.

In selecting metal compounds for impregnating the precursor fibers, nitrates and chlorides have been preferred because of their high degree of solubility in water, and suitable nitrates and chlorides include those of alkaline-earth metals such as calcium, magnesium, barium and strontium, and further include nitrates and chlorides of transition metals such as nickel and iron. Other salts of these metals can of course be selected with due regard to solubility in some solvent which will not attack the organic precursor fibers.

The fluoride can also be provided by a variety of different compounds, so long as the one selected is soluble also in the same or a miscible solvent. Fluorides of potassium, sodium, and ammonia are good examples for use in aqueous solution, and the latter provides the additional advantage that after each reaction with the metal nitrate, the resulting ammonium nitrate will gassify at an elevated temperature so that a subsequent washing operation, prior to calcining of the organic fibers, can be minimized or made unnecessary. Hydrofluon'c acid may also be used instead of a fluoride salt as the second chemical compound which reacts with the metal nitrate to precipitate the necessary metal fluoride. Other soluble fluoridating agents can be used aside from those described above.

EXAMPLE 1 A bundle of rayon tow fibers about 6 to 8 inches long was soaked in distilled water for an hour and one-half to swell the fibers and maximize their ability to absorb the chemical solutions used in the conversion process, and then the bundle was rid of excess water and immersed in a calcium nitrate solution containing 250 grams of Ca(NO -4H O per grams of water for 24 hours. The bundle was then removed from the solution, rid of excess solution by blotting, and immersed in a saturated aqueous solution of NaF for an hour to accomplish complete precipitation of CaF within the rayon fibers. The tow was thereafter rinsed with distilled water to remove all of the soluble nitrate and finally blotted to remove excess water, the tow being subsequently fully dried in a warm oven. This drying step is useful for reversing the swelling of the precursor fibers in order to shrink them and densify the metal fluoride precipitated therein. The calcination step was carried out in a furnace by very slowly increasing the temperature to 400C, at a rate somewhat slower than 100C per hour, and the 400C temperature was held for 4 hours. Thereafter, it was raised gradually to 800C over a 2 hour period and held at 800C for another 7 hours, whereupon the resulting calcium fluoride fibers were removed from the furnace. The fibers thus obtained were tested by immersion in hydrochloric acid to determine that they were in fact fluoride rather than oxide fibers.

EXAMPLE 2 A bundle of rayon fibers, similar to that used in Example l, was then used to make magnesium fluoride fibers in a series of steps similar to those set forth in the first example. In this case a Mg(NO solution of similar concentration was used instead of calcium nitrate, but in the drying step following precipitation and rinsing, the fibers in the bundle were subjected to hot air blowing through them and agitating them to eliminate the tendency of the fibers to adhere to each other. The sintering step was unchanged.

EXAMPLE 3 This example resembles the first example, except that absorbent cotton was used as the precursor and resulted in curly calcium fluoride fibers:

burn off rinse away Cu(NC:)2 2NuF CaFnl I I in cotton 2NuN0i EXAMPLE 4 This example also resembles the first example except that the. sintering step was conducted in a fluoride gas atmosphere to eliminate the possibility of having some of the precipitated metal appear in the form of an oxide instead of a fluoride.

EXAMPLE 5 This example resembles Example 1 except that the chemical solution in which the impregnated fibers are immersed to precipitate calcium fluoride from the impregnating calcium nitrate is ammonium fluoride instead of sodium fluoride. The reaction between the two solutions accordingly precipitates calcium fluoride and produces ammonium nitrate, which is not only soluble in water, but can also be gassified. Therefore, the subsequent washing step to remove this second salt can be either less carefully done, or else eliminated entirely since the subsequently applied heat will drive off the ammonium nitrate formed in the reaction. Of course, any remaining ammonium fluoride still has to be washed away prior to the sintering step.

we. in;

1. A process for producing high tensile strength metal fluoride fibers comprising the steps of impregnating organic material precursor fibers with a compound comprising a soluble salt of one or more of the metals, calcium, magnesium, strontium, barium, nickel and iron;

precipitating insoluble fluoride of the metal within the precursor fibers by introducing a second soluble fluoride solution which will yield said metal fluoride plus another salt; removing said other salt and the remaining second soluble salt from the precursor fibers, drying said precursor fibers containing said metal fluoride, and raising the temperature of said organic material in a gaseous atmosphere to a level sufficient to degrade it but below the melting point of the metal fluoride, the temperature being raised at a slow enough rate to avoid ignition of the organic material.

2. The process set forth in claim 1, wherein said second soluble fluoride is selected such that said another yielded salt is water soluble, and said removing step comprising washing said fibers.

3. The process set forth in claim 1, wherein said second soluble fluoride is selected such that said another yielded salt will gassify and be driven off when the temperature of the precursor fibers is raised.

4. The process set forth in claim 1, wherein said second soluble fluoride is ammonium fluoride.

5. The process set forth in claim 1, wherein the temperature of the organic material is raised in an atmosphere of fluoride gas. 

2. The process set forth in claim 1, wherein said second soluble fluoride is selected such that said another yielded salt is water soluble, and said removing step comprising washing said fibers.
 3. The process set forth in claim 1, wherein said second soluble fluoride is selected such that said another yielded salt will gassify and be driven off when the temperature of the precursor fibers is raised.
 4. The process set forth in claim 1, wherein said second soluble fluoride is ammonium fluoride.
 5. The process set forth in claim 1, wherein the temperature of the organic material is raised in an atmosphere of fluoride gas. 